The present invention relates to optical communication, more particularly to optical code division multiple access (“CDMA”) communication employing differential encoding and detection.
In the past, numerous communications schemes have been developed to increase data throughput, decrease error rates and generally improve performance of the communications channel. In frequency division multiple access (“FDMA”), different data streams are assigned to distinct channels at different frequencies of the transmission band. In time division multiple access (“TDMA”), different data streams are assigned to different timeslots in a single frequency of the transmission band. FDMA and TDMA can be quite limited in terms of the number of users and/or data rates they can support for a given transmission band.
One particularly effective communications scheme that has supplanted FDMA and TDMA in many communication architectures is CDMA. CDMA is a form of spread spectrum communications that enables multiple data streams or channels to share a single transmission band at the same time. The CDMA format is akin to a cocktail party in which multiple pairs of people are conversing with one another at the same time in the same room. As anyone who has been in such a situation understands, it can be very difficult to hear the other party in a conversation if there are many conversations occurring simultaneously. For instance, if one pair of speakers is excessively loud, their conversation will drown out the other conversations. If different pairs of people are speaking in the same language, it is possible that the dialog from one conversation will bleed into other conversations in the same language, causing miscommunication. In general, the cumulative background noise from all the other conversations makes it harder to hear the other party speaking. The goal is to find a way for everyone to communicate at the same time so that each pair's conversation, i.e., “signal,” is clear while minimizing the “noise” of the other pairs' conversations.
The CDMA multiplexing approach is well known and is explained in detail in the book “CDMA: Principles of Spread Spectrum Communication,” by Andrew Viterbi, which was published in 1995 by Addison-Wesley and which is hereby expressly incorporated by reference herein. While the details of CDMA operation are best left to Viterbi's text, it is important to understand some basic CDMA concepts. In CDMA, the bandwidth of the data to be transmitted (user data) is much less than the bandwidth of the transmission band. Unique “pseudonoise” keys are assigned to each channel in a CDMA transmission band. The pseudonoise keys are selected to mimic Gaussian noise (e.g., “white noise”) and are also chosen to be maximal length sequences in order to reduce interference from other users/channels. One pseudonoise key is used to modulate the user data for a given channel. This is equivalent to assigning each pair of partygoers a different language to speak.
During modulation, the user data is “spread” across the bandwidth of the CDMA band. That is, all channels are transmitted at the same time in the same frequency band. This is equivalent to all pairs of partygoers speaking at the same time. The introduction of noise and interference from other users during transmission is inevitable (collectively referred to as “noise”). Due to the nature of the pseudonoise key, the noise is greatly reduced during demodulation relative to the user's signal. This is the case because when a receiver demodulates a selected channel, the data in that channel is “despread” while the noise is not despread. Thus, the data is returned to approximately the size of its original bandwidth, while the noise remains spread over the much larger transmission band. Power control for each user can also help to reduce noise from other users. Power control is equivalent to lowering the volume of a loud pair of partygoers.
CDMA has been used commercially for years in wireless telephone (“cellular”) and other communications systems. Cellular systems typically operate between 800 MHz and 2 GHz, although individual frequency bands may only be a few megahertz wide. One attractive feature of cellular CDMA is that theoretically there is no hard limit to the number of users in a given bandwidth, unlike FDMA and TDMA. Adding more users to the transmission band merely means that there is more noise to contend with. However, as a practical matter, there is some threshold point at which the “signal to noise” ratio becomes unacceptably noisy. This signal to noise threshold places real constraints on commercial systems in terms of the number of paying customers and/or data rates it can support. Therefore, engineers and scientists continually seek to improve CDMA systems by improving the signal to noise ratio.
Recently, CDMA has seen increasing use in optical communications networks. Optical CDMA employs the same general principles as cellular CDMA. Unlike cellular CDMA, optical CDMA signals are modulated at optical frequencies. Regardless, the signal to noise ratio for optical CDMA is just as important as in cellular CDMA. In the past, optical CDMA has employed on-off keying (“OOK”) as part of the encoding and decoding process. However, it is desirable to develop new encoding and decoding technologies that enhances the signal to noise ratio.